Known in the past has been an internal combustion engine provided in an exhaust passage with an electrochemical reactor provided with a ion-conductive solid electrolyte layer, an anode layer arranged on a surface of the solid electrolyte layer, and a cathode layer arranged on a surface of the solid electrolyte layer (for example, JP 2009-138522 A and JP 2008-110277 A). In such an electrochemical reactor, if electric current is provided so as to flow from the anode layer through the solid electrolyte to the cathode layer, the NOX is reduced to N2 and purified on the cathode layer.
In JP 2009-138522 A, the larger the flow rate of the exhaust gas, the larger the current supplied to the electrochemical reactor is made. If increasing the current supplied to the electrochemical reactor, the amount of NOX able to be removed per unit time becomes greater, and therefore by changing the magnitude of the supplied current in accordance with the flow rate of the exhaust gas, it is considered possible to suitably remove NOX regardless of the flow rate of the exhaust gas.
Further, in JP 2008-110277 A, the cathode layer is configured to enable it to hold NOX. Due to this, even if oxygen, which obstructs the chemical reaction of NOX, is excessively present in the cathode layer, NOX is selectively held at the cathode layer and accordingly it is considered possible to efficiently remove NOX.